I generally lump balance issues into a few categories.
Weapons - I mostly think Axe and Falchion and Dual Hammers as being OP, and Shield weapons need buffs. For the first two; I think reduce the damage a bit, and for the second I’d . . . well, maybe reduce cleave some. They fall off hard in modded difficulties, as it is. Shield weapons mostly just need slightly better mobility - not great, but like 2 effective dodges and a x1 multiplier. As it is, they are NOT good defensively, because blocking isn’t a good defensive strategy, and they’re arguably the most dangerous weapons against monsters because dodging their attacks is so difficult to do consistently. Also, I don’t know why things like the 1-Handed Swords don’t all have 6 dodges.
Talents - There are a few that stick out for me - the 50% damage reduction ones for Slayer and Zealot, and Handmaiden’s 3-second invis after Dash. I’d just maybe make the first two have a lesser reduction (35%? I dunno), or make them baked-into the class, because unless they’re awful then they’re still always going to be auto-picks. For Handmaiden, firstly - we need Bladedancer to work, though it might be stupidly overpowered - we need to at least able to use it and have it do SOMETHING. The 3-second invis could just break on attacking, which seems fair. Also, maybe tweak WHC’s level 25 talents. Almost no one ever takes anything but cooldown reduction, because the other two options are so weak.
Foot Knight - he just needs some buffs. Honestly, I feel like he should have the extra cleave, rather than Merc. Merc gets attack speed, and ways to buff his power in combat, and is the only medic - it’d be fair to cut his power down a bit and give it to FK.
Temp Health - I think the idea is good, but it needs to be a bit more limited, because it just trivializes the content so hard. Maybe its decay could ramp up over time, so you can’t just build it up and sit on it all game. Or the amounts you can get could be clipped. Maybe an internal cooldown?
Regarding a new difficulty, I would personally like to see some of the mutators incorporated; like, occasionally enemies do a Tzeentch split, but not every time - it means you’ll just have to be on-guard for when it does happen. Have suicide Skaven mixed into the regular game. Enemy health and spawns could be increased some, but depending on that alone won’t be too much fun, IMO. I do like how things like Onslaught add more shielded enemies, so you have to think about how best to take them down and can’t just spam left-click into hordes mindlessly.
I also think new enemies with features that counter some of our most commonly-used tactics would do a lot to increase difficulty. Standardbearers might be good for that, when WoM arrives. We’ll have to see.